Graphene Domains Research Articles

Tuning the electronic structure and thermodynamic properties of hybrid graphene-hexagonal boron nitride monolayer

A hybrid system which comprises in-plane graphene and hexagonal boron nitride (h-BN) has sparked intense research due to its tailorable and promising physical properties. We have investigated by first-principle calculations the effect of graphene or h-BN domain size on the atomic geometries, electronic properties and thermodynamic properties of graphene/h-BN hybrid systems with a zigzag interface. Lattice dynamics calculations produced no imaginary frequencies suggesting that the structure is dynamically stable regardless of the graphene (h-BN) domain size. However, the band gap and the thermodynamic properties have been demonstrated to be sensitive to the graphene (h-BN) domain size. The band gap, tunable up to 1.01 eV, decreases with the graphene domain size. The Debye temperature responds differently: it increases with the size of the graphene domain. The thermodynamic properties of the hybrid system are bounded by the values for the graphene and h-BN and can be tuned to converge to either the value of graphene or h-BN depending on the size of graphene domain. Detailed chemical and physical insights revealed by this study can pave a way for a more rational design of future nano-devices with optimized electronic and thermal properties.

Amorphous state of sp2 solid carbon

Two-mode valence electron configuration of carbon atoms lays the foundation of the unique two-mode amorphous state of the carbon solid. From the fundamentals of solid-state physics, sp3 and sp2 amorphous carbons are two different species characterized by conceptually different short-range orders, namely, groups of tetrahedrally bonded sp3 configured atoms and nanoscale size-restricted sp2 graphene domains framed with heteroatom necklaces. Molecular character of sp2 amorphics is provided with chemical reactions accompanying the solids amorphization in due course of the enforced fragmentation.

A Monolayer Composite of h-BN Doped by a Nano Graphene Domain: As Sensitive Material for SO2 Gas Detection

The development of two-dimensional materials with gas-sensitive properties has been a research focus in the field of materials science. Considering the excellent properties of graphene and hexagonal boron nitride, a monolayer composite is put forward and its potential for gas sensing has been investigated by density functional theory. Compared to common gases in the atmosphere and three other toxic gases, sulfur dioxide tends to have a strong mutual effect with this composite. Together with electronic analyses, it is proved that the composite presents excellent sensitivity toward sulfur dioxide molecules and the conductivity can be used as a vital parameter to diagnose the status of gas sensor. What’s more, a co-adsorption model demonstrates a favorable performance in high moisture working environment. Thus, we can reasonably draw the conclusion that the presented composite could be an eligible candidate for sulfur dioxide gas detection.

Quantum dots in AA-stacked bilayer graphene

While electrostatic confinement in single-layer graphene and AA-stacked bilayer graphene is precluded by Klein tunneling and the gapless energy spectrum, we theoretically show that a circular domain wall that separates domains of single-layer graphene and AA-stacked bilayer graphene can provide bound states. Solving the Dirac-Weyl equation in the presence of a global mass potential and a local electrostatic potential, we obtain the energy spectrum of these states and the corresponding radial probability densities. Depending on the mass potential profile, regular bound states can exist inside the quantum dot and topological bound states at the domain wall. Controlling the electrostatic potential inside the quantum dot enables the simultaneous presence of both types of states. We find that the number of nodes of the radial wave function of the regular bound states inside the quantum dot is equal to the radial quantum number. The energy spectra of the bound states display anticrossings, reflecting coupling of electron- and holelike states.

Millimeter sized graphene domains through in situ oxidation/reduction treatment of the copper substrate

In chemical vapor deposition of graphene, low nucleation density and large domain sizes are desirable. In the particular case of growth on copper, throughout recent years, several works have reported the oxidation of the substrate through various means, which leads to a reduced nucleation density. In this work, an in situ oxidation and reduction treatment is explored, establishing its connection with the removal of carbon impurities from the copper bulk, as well as clarifying the role of these impurities as graphene nucleation sites. The extent of the contribution of these impurities to the growth of graphene is inferred by experiments using carbon-13 methane feed. The effect of the oxidation and reduction treatment on the substrate’s surface roughness is also addressed. Lastly, the synthesis recipe based on this treatment is supplemented with a growth stage splitting, an increased argon flowrate and a short etching stage, resulting in millimeter sized hexagonal graphene domains (diameters 2–3 mm) with improved electrical transport properties. Thus, the deposition process presented here stands out on account of its streamlined nature, which makes it an attractive candidate for industrial scale-up.

Influence of Substrates on Conductivity of Thin Carbon Films Deposited by Methane Plasma and Subsequent Annealed

The properties of thin carbon films obtained by the deposition of carbon in CH4 - plasma on the surface of various substrates (Si, SiO2 and quartz glass) and subsequent annealing at 650 to 800 °C were investigated. The sizes of graphene domains from 5 to 15 nm from Raman spectra were determined which increases with rising temperature on synthesis of carbon films. Also, sizes of graphene domains were estimated based on the electrical conductivities of obtained carbon films, and their values were equal ∼10.8 nm. The Efros-Shklovskii variable-range hopping (ES-VRH) conduction mechanism is replaced by percolation conductivity with increasing temperature from 650 to 800° C. When using Si substrates, the hopping mechanism was not observed at all temperatures.

Heteroatom necklaces of sp2 amorphous carbons. XPS supported INS and DRIFT spectroscopy

In the case of sp2 amorphous carbons, formed by nanoscale graphene domains framed by heteroatom necklaces (basic structure units in total), inelastic neutron scattering and photoabsorption ensure studying vibrational spectra of the latter mainly. Examination of a set of samples of the highest carbonization rank representing natural substances (shungite carbon, anthraxolite, and anthracite), technical graphenes (laboratory reduced graphene oxides), and industrial products (carbon blacks) has shown, that while INS spectra revealed the hydrogen-enriched component of these necklaces only, the DRIFT spectra exhibited two components determined by both hydrogen and oxygen compositions. Taking together, INS and DRIFT spectra exhibit methine groups, typifying the hydrogen component of all natural amorphics as well as hydroxymethyls and methyls doing the same job for the studied technical graphenes, and hydroxyfurans - for carbon blacks. A comparable analysis of DRIFT and XPS spectra has allowed a reliable personification of the oxygen functional groups compositions. Based on the results obtained, a set of reliable molecular models of the amorphics’ basic structural units has been proposed.

Energetically stretching proteins on patterned two dimensional nanosheets

Driven by increasing interests in sequencing proteins, several single molecule methods (such as the measurement of electron tunneling currents through each amino acid) have been recently proposed and demonstrated in experiment. While promising, they are still in early stages and challenges such as the lack of protein stretching and large thermal fluctuations of amino acids remain to be solved. Here, we show that a patterned two dimensional (2D) in-plane heterostructure comprising both graphene and hexagonal boron nitride (hBN) domains can be utilized to stretch the intrinsically disordered Aβ42 protein because of the chemical potential difference between the graphene and hBN surfaces. Additionally, thanks to the physisorption of the flexible Aβ42 protein on the 2D surface, thermal fluctuations of amino acids are substantially reduced. Therefore, this stretched protein conformation with amino acids adsorbed on the 2D surface may facilitate protein sequencing using the high resolution atomic force microscopy (AFM) or scanning tunneling microscope (STM), with improved signal-to-noise ratios. We further show that the 2D-heterostructure-based protein electrophoresis can help detect the phosphorylation in a single protein molecule. We expect that the patterned 2D heterstructure could serve as a new versatile platform for various single-protein analyses.

In Situ Monitoring of Growth of Vertically Stacked h-BN/Graphene Heterostructures on Ni Substrates and Their Interface Interaction

Vertically stacked hexagonal boron nitride (h-BN)/graphene heterostructures present potential applications in electronic, photonic, and mechanical devices, and their interface interaction is one of the critical factors that affect the performances. In this work, the vertical h-BN/graphene heterostructures with high coverage are synthesized by chemical vapor deposition (CVD) of h-BN on Ni substrates followed by segregation growth of graphene at the h-BN/Ni interfaces, which are monitored by in situ surface microscopy and surface spectroscopy. We find that h-BN overlayers can be decoupled from Ni substrates by the graphene interlayers. Furthermore, the h-BN domain boundaries exhibit a confinement effect on the graphene interlayer growth and the lower graphene domains are limited within the upper h-BN domains. This work provides new insights into the formation mechanism and interface interaction of the vertical heterostructures.

On the Edge of Bilayered Graphene: Unexpected Atomic Geometry and Specific Electronic Properties.

The tendency of bilayered graphene edges to connect with each other allows to create hollow sp2-hybridized material with specific electronic properties. However, the unknown geometry of the formed edges hinders further investigation. Here we show that a closed bigraphene edge can be represented as a connection of generally misoriented graphene domains with topological defects and can be further described by grain boundary theory. The energy dependence of closed edges of commensurate twisted bilayered graphene is derived for any twist angle and edge orientation. Our findings allow to predict what particular edge types appear in the bilayered graphene holes and explain the structure of the connected bilayered graphene edges, which are often observed in the experiment.

Wafer-scale fabrication of single-crystal graphene on Ge(1 1 0) substrate by optimized CH4/H2 ratio

The synthesis of high-quality wafer-scale single-crystal graphene is indispensable for the commercial applications of graphene-based electronic and photonic devices. Previously large-scale single-crystal graphene was successfully acquired by various growth optimization whereas the methods vary from lab to lab due to the complicated process of chemical vapor deposition. Here we present an approach to fabricate monolayer single-crystal graphene on 4-inch Ge(110) wafer by tuning CH4/H2 ratio precisely. By directly monitoring the growth process of graphene via atomic force microscopy, it is found that under high CH4/H2 ratio condition new graphene domains ceaselessly nucleate and enlarge during the whole growth process, while under low CH4/H2 ratio condition, the density of graphene domains is saturated at early stage, which causes the formation of single-crystal graphene. Terahertz time domain spectroscopy shows that the average carrier mobility of single-crystal graphene wafer is as high as 26351 cm2/V·s, comparable to that of the exfoliated graphene. This work provides an efficient and reliable approach to fabricate wafer-scale single-crystal graphene for microelectronics and optoelectronics applications in the future.

Soliton-Dependent Electronic Transport across Bilayer Graphene Domain Wall.

Layer-stacking domain wall in bilayer graphene is one type of topological defects that can greatly affect the electronic properties of bilayer graphene and therefore lead to nontrivial transport behaviors. An outstanding question on the layer stacking domain wall is how the electrons hop between two adjacent stacking domains. Here we report the first experimental observation of electronic transport across bilayer graphene domain walls by combining near-field infrared nanoscopy and scanning voltage microscopy techniques. We observe markedly different electron transport behaviors across the tensile- and shear-type domain walls. The tensile-type domain wall is highly reflective of low-energy incident electrons, but becomes more transparent when the electron density and the Fermi energy are increased by electrostatic gating. In contrast, the shear-type domain wall is always highly transparent at different gate voltages. Such soliton-dependent electronic transport can open up new routes to engineer novel nanoelectronic devices based on layer-stacking domain walls in bilayer graphene.

Precise lateral control of graphene via living zigzag edges

From chemical vapor deposition grown film to nanoscale lithographic devices, etching is a critical procedure to define two-dimensional (2D) materials’ geometry and edge type. Many reported etching procedures could potentially damage the activity of the edge, while a living edge is capable to heal and regrow back. Here we successfully demonstrated a continuous growth-etching-regrowth procedure to achieve the precise lateral control of graphene basal plane. The etching process leaves graphene with dominant zigzag edges. Up to 85% of the new graphene domains can be regrown from the living edges, replicating the graphene’s crystallographic orientation before the etching. At 1050 °C, the etching and regrowth can be linearly controlled at a rate of −2.7 μm/min and 2.4 μm/min, respectively. This represents a promising technique for precisely tailoring the lateral structure of graphene with selective edge type.

Extraction of Graphene Nanostructures from Colocasia esculenta and Nelumbo nucifera Leaves and Surface Functionalization with Tin Oxide: Evaluation of Their Antibacterial Properties.

Expeditious evolution of antimicrobial resistance in recent years has been identified as a growing concern by various health organizations around the world. Herein, facile and environmentally benign production of highly antibacterial carbonaceous nanomaterials from Colocasia esculenta and Nelumbo nucifera leaves is reported. After carbonization and oxidative treatment, smaller graphene domains are formed in Colocasia esculenta derivatives, whereas larger sheetlike structures are observed in the case of Nelumbo nucifera. Smaller particle size makes quantum confinement effects more prominent, as is evident in fine-tuning of the photoluminescence emission after each stage of treatment. The influence of precursor materials on the antibacterial properties of the nanosystems is also demonstrated. When microbiocidal activity was tested against model bacteria Pseudomonas aeruginosa, the nanocomposite derived from Colocasia esculenta leaves showed higher activity than the antibiotic drug clarithromycin (control) with a measured zone of inhibition of 40±0.5 mm. This is one of the highest values reported in comparison with plant-based carbon-silver nanosystems. Quantitative analysis revealed that the nanocomposite obtained from Colocasia esculenta leaves has antimicrobial efficacy equivalent to those of commercial antibiotic drugs and is able to eradicate bacteria at much lower concentrations than that obtained from Nelumbo nucifera leaves.

Identification of Semiconductive Patches in Thermally Processed Monolayer Oxo-Functionalized Graphene.

The thermal decomposition of graphene oxide (GO) is a complex process at the atomic level and not fully understood. Here, a subclass of GO, oxo‐functionalized graphene (oxo‐G), was used to study its thermal disproportionation. We present the impact of annealing on the electronic properties of a monolayer oxo‐G flake and correlated the chemical composition and topography corrugation by two‐probe transport measurements, XPS, TEM, FTIR and STM. Surprisingly, we found that oxo‐G, processed at 300 °C, displays C−C sp3‐patches and possibly C−O−C bonds, next to graphene domains and holes. It is striking that those C−O−C/C−C sp3‐separated sp2‐patches a few nanometers in diameter possess semiconducting properties with a band gap of about 0.4 eV. We propose that sp3‐patches confine conjugated sp2‐C atoms, which leads to the local semiconductor properties. Accordingly, graphene with sp3‐C in double layer areas is a potential class of semiconductors and a potential target for future chemical modifications.

Nanoscale strain engineering of giant pseudo-magnetic fields, valley polarization, and topological channels in graphene.

The existence of nontrivial Berry phases associated with two inequivalent valleys in graphene provides interesting opportunities for investigating the valley-projected topological states. Examples of such studies include observation of anomalous quantum Hall effect in monolayer graphene, demonstration of topological zero modes in "molecular graphene" assembled by scanning tunneling microscopy, and detection of topological valley transport either in graphene superlattices or at bilayer graphene domain walls. However, all aforementioned experiments involved nonscalable approaches of either mechanically exfoliated flakes or atom-by-atom constructions. Here, we report an approach to manipulating the topological states in monolayer graphene via nanoscale strain engineering at room temperature. By placing strain-free monolayer graphene on architected nanostructures to induce global inversion symmetry breaking, we demonstrate the development of giant pseudo-magnetic fields (up to ~800 T), valley polarization, and periodic one-dimensional topological channels for protected propagation of chiral modes in strained graphene, thus paving a pathway toward scalable graphene-based valleytronics.

Effects of the oxide layer with different thicknesses on copper substrate on depressing graphene nucleation by low pressure chemical vapor deposition

The copper foils covered by oxide layers with different thicknesses, i.e. the naturally-oxidized substrate and the intentionally-oxidized substrate were prepared to fabricate graphene by low pressure chemical vapor deposition. To prevent the oxide layer being reduced in the heating stage, N2 was introduced into the growth chamber. The results indicated that although the graphene nucleation density on both substrates was reduced effectively, that on the intentionally-oxidized substrate was lower. Meanwhile, the graphene domains on the intentionally-oxidized substrate were monolayer with better crystal quality as well as larger size while that on the naturally-oxidized substrate possessed poor quality caused by the coexistence of monolayer and few-layer graphene. Based on the analysis of the morphological changes of the substrate surface caused by the oxide layer in different growth stages, the mechanism of the oxide layer affecting graphene nucleation and growth was investigated. And an easier and effective way to fabricate large-sized graphene domains with good quality was also proposed.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Bipolar magnetic semiconductor properties and spin-dependent Seebeck effects induced by nanoscale graphene domains doped into armchair boron nitride nanoribbons

Inspired by the synthesis of nanometer-scale graphene quantum dots via a doping process in boron nitride (BN) sheets (Adv. Mater. 2019, 1805778), we investigated the magnetic and thermal spin transport properties of armchair BN nanoribbons containing nanoscale graphene domains. It was found that both M-QB and M-QN structures were bipolar magnetic semiconductors (BMSs). Two spin caloritronics devices were designed on the basis of the 15-6B and 15-6N nanoribbons. Spin-dependent Seebeck effects, spin-dependent Seebeck diode effects and thermoelectric diode effects were observed in these devices. This work provides theoretical guidance for the design of new spin caloritronics devices.

Effect of copper substrate oxidation and crystals orientations on nucleation of triangular graphene domains

In graphene fabrication by chemical vapor deposition (CVD), substrate crystal structure and orientation play an essential role in the formation of graphene shapes. This work gives experimental evidence and creates a better understanding of the role of oxygen on graphene domains’ shape. It explains for the first time how triangular-shaped graphene is formed on copper. The experimental results verify that the growth of the triangular graphene is self-limited, and it is as a result of Cu substrate oxidation. In this work, the copper substrate and graphene were studied before and after growth by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoemission spectroscopy (XPS). The results reveal that the triangular-shaped graphene is multilayered and covalent with oxygen. It shows that this type of graphene is grown on Cu2O grains of (111) facets. This work confirms that the nucleation mechanism and the angles of triangular shape graphene are affected by structure of substrate crystals, facets, and orientations.